In general, photographic materials containing a silver halide emulsion layer are subject to various outside pressures. For example, negative films for general photography are apt to be bent when rolled in a cartridge or loaded into a camera, or pulled or scratched with a carriage part of a camera on film feeding. Sheet films such as printing films and direct radiographic films for medical use are often bent when handled by hand. When handled in daylight conveying equipment or high-speed changers, photographic materials are brought into contact with metallic or rubber parts under strong pressure. Further, all kinds of photographic materials receive great pressure when trimmed or finished.
Pressure thus applied to a photographic light-sensitive material is transmitted to the silver halide grains through gelatin, a binder for the silver halide grains, or an other high-molecular weight substance which functions as a mediator. It is known that application of pressure to silver halide grains causes blackening irrespective of exposure amount or desensitization. For the details of these phenomena, reference can be made, e.g., in K. B. Mather, J. Opt. Soc. Am., Vol. 38, p. 1054 (1948), P. Faelens and P. de Smet, Sci. et Ind. Photo., Vol. 25, No. 5, p. 178 (1954), and P. Faelens, J. Photo. Sci., Vol. 2, p. 105 (1954).
There has therefore been a demand for a photographic light-sensitive material whose photographic performance is unaffected by pressure. Susceptibility to pressure is difficult to control, particularly in photographic materials in which the amount of a binder is reduced so as to improve suitability for rapid processing.
In general, there is an unfavorable correlation between photosensitivity and pressure sensitivity. That is, as photosensitivity increases, pressure sensitivity also increases.
Moreover, a sensitizing dye promotes the tendency of silver halide grains to cause fog when subjected to pressure. If a large quantity of a sensitizing dye is used for color sensitization in an attempt to increase light absorption and thereby to increase sensitivity, it follows that blackening due to pressure application becomes remarkable. As a means to avoid this disadvantage, it is known to incorporate a plasticizer for polymers or emulsions or to reduce the silver halide/gelatin ratio to thereby prevent applied pressure from reaching the silver halide grains.
Known plasticizers include heterocyclic compounds as disclosed in British Patent 738,618, alkyl phthalates as disclosed in British Patent 738,637, alkyl esters as described in British Patent 738,639, polyhydric alcohols as disclosed in U.S. Pat. No. 2,960,404, carboxyalkyl cellulose as disclosed in U.S. Pat. No. 3,121,060, paraffin and carboxylic acid salts as disclosed in JP-A-49-5017 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), and alkyl acrylates and organic acids as disclosed in JP-B-53-28086 (the term "JP-B" as used herein means an "examined published Japanese patent application").
Since addition of a plasticizer causes a reduction in the mechanical strength of an emulsion layer, there is a limit to allowable amount of a plasticizer that may be added. Further, an increase in the gelatin amount results in retardation of development, which is unfavorable for a photographic material which is to be subjected to rapid processing. Accordingly, sufficient improvement in pressure characteristics can hardly be obtained by either of the above-described means.
On the other hand, tabular grains provide high optical density with a reduced silver amount because of their high covering power per unit area as described in U.S. Pat. Nos. 4,434,226, 4,439,520, and 4,425,425. In addition, they have a large surface area per unit volume and are accordingly capable of adsorbing a larger quantity of a sensitizing dye in spectral sensitization, thus exhibiting a higher light capturing ability. These advantages of tabular grains can be best used with a sensitizing dye in an amount of 60% or more, preferably 80% or more, and more preferably 100% or more, of the saturation adsorption. As previously stated, however, pressure sensitivity increases with the amount of the sensitizer present. Additionally, the shape of the tabular grains makes them likely to deform on the application of an outer force. For these reasons, use of tabular grains does not achieve particularly satisfactory improvement in pressure characteristics.